cmake: use target_include_directories for modern cmake

Apply `target_include_directories` when CMAKE_VERSION >= 2.8.12

Author: AndreyG

CMakeLists.txt

@@ -207,7 +207,6 @@ FILE(STRINGS "include/git2/version.h" GIT2_HEADER_SOVERSION REGEX "^#define LIBG
 STRING(REGEX REPLACE "^.*LIBGIT2_SOVERSION ([0-9]+)$" "\\1" LIBGIT2_SOVERSION "${GIT2_HEADER_SOVERSION}")
 
 # Find required dependencies
-INCLUDE_DIRECTORIES(src include)
 
 IF (SECURITY_FOUND)
   # OS X 10.7 and older do not have some functions we use, fall back to OpenSSL there
@@ -619,6 +618,13 @@ TARGET_LINK_LIBRARIES(git2 ${GSSAPI_LIBRARIES})
 TARGET_LINK_LIBRARIES(git2 ${ICONV_LIBRARIES})
 TARGET_OS_LIBRARIES(git2)
 
+IF (${CMAKE_VERSION} VERSION_LESS 2.8.12)
+	INCLUDE_DIRECTORIES(src include)
+	SET(LIBGIT2_INNER_DIRECTORIES_ARE_INCLUDED TRUE)
+ELSE()
+	TARGET_INCLUDE_DIRECTORIES(git2 PRIVATE src PUBLIC include)
+ENDIF()
+
 # Workaround for Cmake bug #0011240 (see http://public.kitware.com/Bug/view.php?id=11240)
 # Win64+MSVC+static libs = linker error
 IF(MSVC AND GIT_ARCH_64 AND NOT BUILD_SHARED_LIBS)
@@ -688,6 +694,10 @@ IF (BUILD_CLAR)
 		${CLAR_PATH}/clar.c
 		PROPERTIES OBJECT_DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/clar.suite)
 
+	IF (NOT LIBGIT2_INNER_DIRECTORIES_ARE_INCLUDED)
+		INCLUDE_DIRECTORIES(src include)
+	ENDIF()
+
 	ADD_EXECUTABLE(libgit2_clar ${SRC_H} ${SRC_GIT2} ${SRC_OS} ${SRC_CLAR} ${SRC_TEST} ${SRC_ZLIB} ${SRC_HTTP} ${SRC_REGEX} ${SRC_SSH} ${SRC_SHA1})
 
 	TARGET_LINK_LIBRARIES(libgit2_clar ${COREFOUNDATION_DIRS})